1. Field of the Invention
The present invention relates to a vibration wave driven motor in which a voltage is applied to an electromechanical energy converting element to generate a travelling vibration wave in a vibration member, thereby causing a relative movement to a member in contact therewith by frictional drive.
2. Related Background Art
A vibration wave motor is composed of a vibration member for generating a travelling vibration wave, and a movable member maintained in friction contact therewith. More specifically, the conventional vibration wave motor, for example of a high output type, is constructed in the following manner. The above-mentioned vibration member for generating the travelling vibration wave is composed of an annular elastic member formed for example of stainless steel, on the rear face of which is adhered a group of thin piezoelectric elements formed in the same annular shape. The front face of the vibration member is formed as a hard sliding face by emission of an ultra hard material consisting of tungsten carbide and cobalt, followed by polishing. On the other hand, the movable member to be contacted with the front face of the vibration member is formed by laminating a slidable member on a metallic annular support member composed for example of an aluminum alloy. The slidable member is formed for example by forming a molded cylinder with a heat resistant resin with a glass transition point at least equal to 100.degree. C., such as polyimide (PI), polyamidimide (PAI), polyetherimide (PEI), polyetheretherketone (PEEK), polyethersulfone (PES), polyarylate (PAR), polysulfone (PSF) or aromatic polyamide (PA) filled with a reinforcing material such as carbon fibers, and cutting the molded cylinder perpendicularly to the axis thereof to obtain an annular disk of a thickness for example of 1 mm. The movable member is formed by adhering the thus obtained slidable member of composite resin of the annular disk shape, with a heat-resistant adhesive, to the above-mentioned support member for example of aluminum alloy.
In the relative movement of the vibration member and movable member, either member theoretically maybe selected as the fixed one, and, in this sense, the above-mentioned movable member indicates a member causing relative movement with respect to the vibration member by frictional drive. However, it is conventionally customary to construct the vibration member as fixed, and, for the purpose of clarity, the member maintained in contact with the vibration member will be hereinafter called the "movable member".
In the conventional vibration wave motor, as explained above, the sliding face of the movable member is formed by a slidable member of composite resin, composed of a thermoplastic resin with the glass transition point at least equal to 100.degree. C. as the matrix and reinforced by fiber filling. Such heat-resistant resin has little temperature dependence of the physical properties. Therefore, it is free from loss of torque resulting from the softening of resin and is capable of stabilizing the performance of the motor even at an elevated temperature encountered in the course of motor operation.
Also the addition of a reinforcing filler such as carbon fibers to said thermoplastic resin is, firstly, to constantly stabilize the state of the sliding face of the movable member against the ultra-hard sliding face, consisting of tungsten carbide and cobalt, of the vibration member and to ensure a sufficient abrasion resistance for a prolonged operation with a large load torque, and, secondly, to improve the performance of the vibration wave motor such as the output thereof by increasing the physical properties such as elastic modulus or thermal conductivity of the movable member.
As explained in the foregoing, the conventional vibration wave motor, in which the sliding face of the movable member is formed by a slidable member of reinforced composite resin composed of a heat-resistant thermoplastic resin with the glass transition point at least equal to 100.degree. C. and reinforced with a filler such as carbon fibers, shows a high abrasion resistance of the movable member even in a prolonged frictional drive by the ultra-hard vibration member, and also shows high motor performances such as output and efficiency.
However, such conventional vibration wave motor has been associated with a fluctuation in torque, exceeding 5% of the rated torque when continuously driven under rated conditions, and an improvement in the stability of torque is desirable.
According to the investigations by the present inventors, said fluctuation in torque is attributable to the local unevenness in distribution of the reinforcing fibers added in the slidable member and the unevenness in orientation of said fibers.
Also such conventional vibration wave motor has been associated with an undulation in torque, with the temperature increase in the faces in frictional contact, when continuous drive is started with rated conditions such as 4 kg.cm, 100 rpm.
Furthermore such motor has been associated with so-called "squeaking" resulting from sliding friction in operation without load or with a low load torque, though this drawback is not encountered when the load torque is large.
In addition to the above-mentioned drawbacks related to the characteristics of the vibration wave motor, the cost of the movable member is difficult to reduce because the slidable member is cut out from a molded cylinder. More specifically, the preparation of the movable member requires a series of steps, such as polishing the thus cut-out sliding member and the adhesion face of the support member to a suitable coarseness, then fixing the members by exposing them in a mutually contacted state with heat-resistant adhesive therebetween in an electric furnace for example of 60.degree. C. for several hours, correcting the dimensions by grinding the external and internal walls, and effecting a final finishing of the sliding face by polishing.